1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to a glass-to-metal seal (GMTS) for hermetically sealing an electrochemical cell. The glass-to-metal seal is considered critical because it hermetically isolates the internal environment of a component from the external environment to which the component is exposed. In electrochemical cells powering implantable medical devices, the GTMS hermetically seals the internal cell chemistry from the internal device environment.
2. Prior Art
The glass-to-metal seal of an electrochemical cell consists of a ferrule sleeve secured to an opening in the cell casing, such as in the lid or in the casing body itself. The ferrule supports an insulating glass in a surrounding relationship and the glass in turn seals around the perimeter of a terminal lead. The terminal lead extends from inside the cell to a position outside the casing, and serves as the lead for one of the cell electrodes. Typically the terminal lead is connected to the cathode current collector. The casing including the lid serves as the second terminal for the other electrode, typically the anode. This configuration is referred to as a case-negative design.
To construct a glass-to-metal seal, insulating glass is provided in a ring shape to fit inside the ferrule sleeve or inside an opening in the casing body in a closely spaced relationship. The insulating glass has a hole through its center that receives the terminal lead in a closely spaced relationship. These components are assembled and then heated in a furnace. This heating step causes the glass to soften and flow into intimate contact with the inside of the ferrule and with the perimeter of the terminal lead. When the assembly cools, the insulating glass is bonded to the ferrule and the terminal lead.
Glass-to-metal seals are defined by the coefficient of thermal expansion of the materials of construction. Conventional glass-to-metal seals fall into two main types. The first is a matched seal where the coefficient of thermal expansions of all of the materials of construction are reasonably similar. The other is referred to as a compression seal. In this type, the coefficient of thermal expansion of the ferrule sleeve or of the casing body is higher than that of the insulating glass while the coefficients of thermal expansion of the terminal lead and the insulating glass are substantially the same. Compression type glass-to-metal seals are shown in U.S. Pat. No. 3,225,132 to Baas et al., U.S. Pat. No. 4,053,692 to Dey, U.S. Pat. No. 4,430,376 to Box and U.S. Pat. No. 4,587,144 to Kellerman et al.
The present invention is directed to a reverse mismatched compression glass-to-metal seal where the coefficient of thermal expansion of the insulating glass is less than that of the terminal lead and, the ferrule or casing body has a coefficient of thermal expansion which is substantially similar to or significantly greater than that of the terminal lead. Theoretically, seals made with a terminal lead having a coefficient of thermal expansion greater than the insulating glass should display stress levels that compromise hermiticity.
These and other objects of the present invention will become increasingly more apparent to those of ordinary skill in the art by reference to the following description and the appended drawings.